The molybdenum-based binary oxides CoMoO₄ have been synthesized using the sol–gel method. Thermal analysis (TGA-DTA), X-ray diffraction (XRD), and Rietveld refinement data were used to characterize the nanocrystalline powders that were produced. A simple grinding of the purple β-CoMoO₄ molybdate leads to the formation of the green polymorph α-CoMoO₄ (piezochromism phenomenon). Heating the green α-CoMoO₄ powder to 500 °C at room pressure allows obtaining the purple β-CoMoO₄ variety (thermochromism phenomenon). The T_1/2 transition temperature which corresponds to the transformation of half of the phase α into β is about 435.5 °C. According to the Rietveld refinement results, the refinement parameters showed that the observed and refined XRD patterns are in good agreement and the CoMoO₄ (α and β) monoclinic molybdate samples are totally pure. Scanning electron microscope (SEM) of β-CoMoO₄ powder obtained from cobalt nitrate (G₁ gel) showed a mixture of porous agglomerates with dispersed nanoparticles. Meanwhile, pre-sintering was observed in the powders obtained from cobalt chloride (G₂ gel). Transmission electron microscopy (TEM) analysis of β-CoMoO₄ molybdate indicated quasi-spherical particles with nanometric sizes ranging from 10 to 60 nm. The analysis of the UV–Visible spectra allows us to affirm that the thermochromism phenomenon is due to the displacement of the O^2–→Mo⁶⁺charge transfer band towards high energies.